You wouldn't need an unlimited pot of money to plan out the general main story arcs for the whole show rather than one season at a time. You could write the specific and review later, but there should be a general overview and description of each character done first so the characterisation is fairly consistent and the plot lines interesting and make sense.

Lol Arya being "not Arya" or the Waif would be horrendous writing and make even less sense than the already bad writing that has been happening for Arya from season 5.
Arya's fight with Brienne, the whole stupid Winterfell drama with Sansa until they inevitably kill LF, etc etc - none of it would even come close to the pure fanficbess and absurdity of a plot where the real Arya is dead and the girl we are seeing now is really the Waif. It wouldn't be clever, it would just be pure and utter nonsense.

I will also add that Lyanna Mormont is pretty good here too, brilliant for her age, and her words and delivery inspired some of the other Northern Lords (Manderly, Glover) who probably never would have declared Jon KiTN without her influence.

We haven't really seen a lot of public speaking on the show, except from Dany, but one who is definitely up there is Ned Stark.
He had no qualms whatsoever in publicly sentencing Grwgor Clegane to death and denouncing Lord Tywin and his oration skills were very good in doing so. The wisdom of that move is debateable, but this thread is not the place for that. Its about ability to orate and from what little we saw of Ned doing that, he was very good and probably only second to Dany in show.

That's quite a stretch.
Arya wasn't baiting Sansa. She was annoyed at her for not defending Jon better but the only reason she said that comment (her original response was "nothing") was because Sansa pressed her not to do that and to say what she meant.
And anyway, Arya's comment had a solid basis in fact. Sansa did (and does still) like nice things (and theres nothing inherently wrong with that) and she did like to feel better than others. Empathy, particularly for commoners or the downtrodden in that society, was never Sansa's strong suit, whereas it was Arya's.
However, I do agree Arya's comment was based on the Sansa she knew as a girl, not the woman Sansa.

Varys and Littlefinger are not main characters, never have been. They are secondary characters and important to the plot, but ASOIAF is not and was never meant to be their story. They influence the plot and the main characters, but the story isn't about them.
It is about Jon, Dany, Tyrion, Arya and Bran (possibly Sansa, Jaime and Cersei as well - especially in the show). Thats 8 main characters.
EVERYONE else is a secondary character. The story is not about them. They can serve an important role and influence on the story and main characters, and even have some development themselves, but they are in a supporting role. Their primary place in the story is on how they impact/support the lives of the main characters and the plot.
Varys, LF, Brienne, Hound,Sam, Ned, Cat, Robb, Ser Davos, Ser Jorah, Ser Barristan, Thoros, etc - all supporting characters.

For me personally, I think the show was a reasonable adaptation up until midway through season 4, although still too many unnecessary scenes for my liking which didnt advance plot or characters.
From Season 5 on, show has pretty much been pure fanfic with little making sense from a narrative or characterisation perspective.
I would have:
*Cut Aegon plot completely, just as D&D did. That I feel was a good move. I also would have cut or reduced plot threads related to Aegon, such as Dorne. I wouldnt have introduced the Sand Snakes at all and would never have sent Jaime and Bronn there. I would likely have kept Ellaria as the face of Dorne though but in a minor role (similar to how Grey Worm is face of Unsullied, Tormund of Wildlings, etc) and had her join Dany.
*Shown Jaime's and Cersei's relationship breaking down much sooner (likely Season 4).
*Show Arya making clearer, more pronounced progress in her training in Braavos (season 5) and learning a wider range of skills (not just combat based) from Jaqen as well as the Waif and have her fight with the Waif still happen, but have Arya beat her in a clearer and more believable way.
Have her arrive in Westeros and kill Walder Frey in premiere of Season 7.
*Have Ser Barristan alive and advising Dany through to Season 6 after having revealed some more pertinent information to her, then killed him off end of S6/start of S7 shortly after Tyrion arrives.
*Keep Sansa in Vale longer with LF and learning a lot from him. Dont send her to WF and certainly dont marry her to Ramsay. It would have been better to see her married to the young Lord of the Vale (Robin Arryn) after Lysa died and see her learn how to manipulate him to do what she wants and also gradually learn how to best LF at own game.
Id make loads of other changes, but will expand more later.

I dont think Arya meant Sansa should have beheaded those Lords at all just for disagreeing. She was more upset that Sansa didn't defend Jon more firmly. Her comments about Sansa liking nice things (which was a fair comment and even now, though Sansa has changed, still had truth to it) and about "not if they lose their heads" had more to do with her anger at Sansa for not defending Jon properly. Sansa's defence of Jon was weak and she knew it.
If Arya was serious about beheading them, then the writers have ruined her character beyond belief for the sake of Sansa.
Arya is not an idiot and nor is she just an assassin or warrior with no political skills. I hate how many show only watchers and even book readers say Arya has no leadership skills when in fact she has them in spades. Her methods and drive isnt focused in that direction, but Arya has a lot of qualities needed in a leader and indeed, in the books her character seems to be destined for some type of leader role, albeit likely a non-traditional one. She will likely be a champion of the smallfolk or common people.
Arya is supposed to be smart, cunning, empathetic, ruthless to her enemies and learning the ways of assassination, stealth, manipulation, lie detection, etc. BookArya certainly would be more than capable of handling herself in politics and dealing with the likes of Varys and LF.
It really pisses me off how the showrunners are ruining the essence of Arya by just making her a badass fighter/assassin but not showing her other skills.

Actually, the show would have been even worse if they focused just on Jon and Dany even earlier.
And 5 or 6 (if we add Sansa) is NOT too many 'main' characters. Even 8 (if we add Cersei and Jaime) is manageable in an ensemble tv show.
Game of Thrones would have been so much a better show and told a better story if they hadn't sacrificed characterisations and plot lines because of Jonaerys or showrunner favouritism for specific actors and expanding their roles because of that.
D&D should have just told the story (although still cutting a crapload of side stories and characters from the books) for the main characters, with a little backstory for the more important secondary characters (Brienne, the Hound, Varys, Littlefinger, etc) and used more focused and logical plot lines for the main characters.
D&D ruined Varys, Arya, Tyrion, Littlefinger, Bran, Jaime and Sansa's characters and plot lines for the sake of Jon, Daenerys and Cersei ---> and it really sucks.

Actually GRRM named 5 main characters for the books, not 2. Jon, Daenerys, Tyrion, Arya and Bran.
Personally, I dont believe that ASOIAF has only one core story - it has a few - and Jon and Dany wont necessarily be the only heroes or even necessarily the 'main' heroes by end of it.
Game of Thrones now seems to have mostly one main plot, but that is mostly due to poor planning and horrible writing, not because of any actual story based need to focus almost entirely on Jon and Dany.